Methods for equalizing injection quantity differences between the cylinders of an internal combustion engine detect differences in the injection quantities between the cylinders and optionally equalize these differences. It is precisely for the diagnosis of small injection quantities—in particular where multiple injection is used in diesel engines of pre-injection quantities that lie in the region of a few milligrams—that a diagnosis of injection quantities is crucial for adhering to exhaust emissions.
Such a method is disclosed for example in the unpublished application DE 10 2004 006 293.
It has proven to be the case that, when equalizing injectors by means of the actuator signal, fixed operating levels have to be run in for several seconds, depending on the injection pressure, in order to be able to determine the actuator energy of the respective injector. In the case of active control, the time between the activation of the injector and that of the actuator signal of all the injectors is set to a value stored in the ignition map. Since a minimum duration of activation of the injector is required for control, this corresponding to a minimum injection quantity, then through the use of multiple injection (one main and several pre- and post-injections), the operating range in which control is possible decreases. Multiple injections are successfully employed for reducing emissions as well as for reducing combustion noise. The following example should illustrate the problem: for a load point A, a quintuple injection of respectively one milligram per stroke is required. However, control for equalizing the cylinders requires a minimum quantity of three milligrams per stroke.
Detection of and control in response to the actuator signal is required precisely when the conditions of the injector change such as, for example, through heating up after a cold start. In this case, it is possible that after a cold start the vehicle will be operated only in a low load range so that there is no possibility of equalizing the cylinders. In such a drive cycle (cold start and subsequent travel in a low load range) the cooling-water temperature changes from the ambient temperature to the target temperature of approx. 90° C. As it does so, the injection quantity can change by approx. 40% due to the heating-up process. Control of the injector in response to the actuator signal is always required once the operating conditions have changed (temperature, rail pressure, etc.).